Median Nerve Injury: Index Finger Flexion Sign

The inability to flex the index finger is a specific clinical sign often associated with damage to the median nerve.

Role of the Median Nerve

The median nerve controls several muscles in the forearm and hand. Specifically, it innervates the flexor digitorum superficialis (FDS) muscle, which is a primary flexor of the finger at the proximal interphalangeal (PIP) joint. It also contributes to the flexion of the index finger at the metacarpophalangeal (MCP) joint via the first lumbrical muscle.

• Injury to the median nerve can impair the function of the FDS muscle.
• This impairment leads to difficulty or inability in flexing the index finger, particularly noticeable at the PIP joint.

Evaluating Other Options

The other options are less likely to cause this specific symptom:

• Ulnar nerve injury: Primarily affects the little and ring fingers, and hand intrinsic muscles responsible for abduction, adduction, and thumb opposition.
• Radial nerve injury: Affects wrist and finger extension (causing wrist drop) and sensation on the back of the hand. It does not directly impact finger flexion.
• Dupuytren's contracture: A condition causing thickening of the palmar fascia, leading to finger flexion contractures (usually ring and little fingers), but not an *inability* to actively flex due to nerve damage.

Conclusion

Therefore, the inability to flex the index finger strongly suggests a lesion affecting the median nerve.